1. Field of Invention
The present invention generally relates to machines or devices for stringing sports racquets and more particularly is concerned with a tensioning device for stringing sports racquets having a rotatable string guide for maintaining a constant string line of action to a string gripper of the tensioning device.
2. Description of Prior Art
There have been various types of devices and machines for stringing sports racquets which utilize a manual or electrical means to apply tension to the racquet strings by pulling and stretching the string. Some of these devices include an automatic means for stopping or braking the pulling action when the desired string tension is achieved by using a predetermined tension setting device, such as those disclosed in U.S. Pat. No. 3,441,275 to Held, U.S. Pat. No. 3,918,713 to Kaminstien and U.S. Pat. No. 5,026,055 to Longeat. Other devices use a visual scale which indicates when the desired tension is achieved, and the stopping or braking action is applied manually such as that disclosed in the U.S. Pat. No. 3,823,609 to Miyagawa.
In the disclosures of the Held and Kaminstien patents, the string tensioner relies on a counter force system where the string force from tensioning the string is applied via a string gripper device to a tension setting mechanism, comprised of a lever arm pivotally mounted to a frame. The lever arm is counter balanced on the end opposite to the string gripper by a counter force, such as that generated by compressing a spring. As the string tension increases, the leverage to the string tension side of the lever arm increases and causes the lever to rotate. After rotating a predetermined amount, based on the tension setting of the string tensioner, a braking device is activated that causes the string tensioner to stop pulling the string. In the event the angle in which the string enters the string gripper device of the string tensioner described in the above referenced prior art is altered due to a different elevation between the clamped end of the string and the tensioning end of the string, the leverage produced by the string tension against the lever arm, of the string tensioning mechanism, which opposes the leverage applied by the compression spring on the opposite end of the lever arm, will be different. Since the predetermined tension setting, which establishes when to activate the braking device is based on a fixed correlation between the leverage produced by the string tension and the leverage produced by the counter balance spring, a difference in the leverage produced by the string tension will cause the braking device to activate prematurely, resulting in less than desired tension in the strings, or will cause the braking device to activate late, causing more than the desired tension in the strings.
In other devices known to exist in the art, such as that disclosed by the Miyagawa patent, the string tensioner involves a string gripper mounted on one side of a tension setting mechanism, comprised of a connecting arm which is mounted to a frame via a shaft and bushing arrangement or a slide and block arrangement, that will only permit linear translation of the connecting arm. As the string is pulled and tensioned, a force sensing device, attached to the pulling side of the connecting arm between the connecting arm and pulling mechanism, senses the force applied to the connecting arm and converts this force into a tension reading. When the force applied to the connecting arm reaches an amount that corresponds to the desired amount of tension in the strings by way of a predetermined tension setting based on a fixed correlation between the measured force on the connecting arm and the tension in the string, a braking device is activated causing the tensioning mechanism to stop pulling the string. Once again, if the line of action of the string tension on the string gripper is at different angles relative to the connecting arm, the correlation between the string tension in the racquet and the force measured by the sensing device will be different causing the string tensioner to stop pulling the string either prematurely or later which will result in less or more tension in the strings than desired.
In all the above cited cases, the angle in which the string enters the string gripper of the string tensioner effects the line of action of the string force on the tension setting mechanism, and will influence correlation between the predetermined tension setting of the string tensioner which achieves the desired tension in the string, and the counter balance force system of the tension setting mechanism, or the correlation between the predetermined tension setting of the string tensioner which achieves the desired tension in the strings and the force sensing system. The counter balance force system and the force sensing system both have a fixed correlation to the tension settings of the string tensioner which sets a reference point as to when to stop tensioning the string in order to achieve the desired tension in the string. In the cases where the string might enter the string gripper at various angles relative to the tension setting mechanism, due to different elevations from the point where the string is clamped relative to where the string enters the string gripper, the line of action of the string force relative to the string gripper will change, and will effect the correlation between the desired string tension and the predetermined setting of the string tension on the string tensioner. In these situations, the braking mechanism will be activated too late or too soon resulting in more or less than the desired tension in the string. In this situation, the racquet stringing machine is said to be out of calibration and requires some adjustment.
The Longeat patent teaches the use of a circular shaped guiding device to guide the string from the racquet into the gripper of the string tensioner. However, the guiding device is permanently attached to the string gripper and tension setting mechanism, and effectively becomes part of the string gripper and tension setting mechanism. Therefore, the line of action of the string force from the racquet to the guiding device will change as the angle of the string entering the guiding device is changed. As mentioned previously, the change to the line of action of the string force on the tension setting mechanism effects the calibration of the racquet stringing machine.
While the prior art stringing devices can be adjusted, or calibrated, to compensate for the different lines of action in which the string force is applied to the string gripper and tension setting mechanism, this adjustment or calibration process, is tedious and inconvenient, and must be done prior to stringing every racquet.
Therefore, there exists a need to improve the current racquet stringing devices to overcome the stated disadvantages associated with how the string is guided to the string gripper and tension setting mechanism of the string tensioner.